Helpful Reply

type about:config in the URL bar and hit Enter
If you see the warning, you can confirm that you want to access that page.
Filter = general.useragent.
Right-click the preferences that are bold, one line at a time, and select Reset,
Then restart Firefox

Your UserAgent string in Firefox is messed up by another program that you installed, and the Facebook website doesn't know you are running Firefox 3.5.10.
[http://en.wikipedia.org/wiki/User_Agent]
type '''about:config''' in the URL bar and hit Enter
''If you see the warning, you can confirm that you want to access that page.''
Filter = '''general.useragent.'''
Right-click the preferences that are '''bold''', one line at a time, and select '''''Reset''''',
Then restart Firefox